NEED FOR PHASE ANGLE MEASUREMENT
Phase angle meters are used to verify the correct connection of three-phase
transformer banks which must be paralleled with an existing electrical bus or
high voltage line. The process of making these measurements is known as
“phasing-out” and is performed before the tie-in is made.
Phase angle measurement is also employed to analyze the operation of AC
synchronous generators and synchronous motors to verify the proper operation
of field regulators and synchronizing equipment.
The Phase angle measurement is used for verifying the proper installation of
medium and high-voltage primary metering equipment and sophisticated
protective relays that receive input from Potential and Current Transformers (PTs
Phase Angle Measurement for Power Factor Determination:
The most important application of the equipment which measures phase angle is
the determination of power factor for conducting electrical system load studies
and power factor correction studies.
The system power factor is equal to the cosine of the phase angle that exists between
the system voltage and current. In the ideal AC electrical system the voltage and current
are in phase. This condition only occurs on systems where the entire load is resistive,
such as electric heat, incandescent lighting, or fluorescent lighting with power factor
corrected ballasts. Electrical utilization equipment such as motors and welders has a
considerable amount of inductance and the inductive reactance (XL which is measured in
International Journal of Emerging trends in Engineering and Development Issue 2, Vol.6 (September 2012)
Available online on http://www.rspublication.com/ijeted/ijeted_index.htm ISSN 2249-6149
Page 475 Measurement of Phase Angle using a PLL ohms) causes the circuit current to lag
the applied voltage. The actual amount, or
number of degrees of lag, depends on the ratio of the Inductive Reactance (XL) in ohms
to the ohmic value of Resistance (R) of the system. Once the system power factor is
known, power factor correction, if desired, can be applied to the system using power
factor correction capacitors or by using synchronous motors, either of which can supply
leading Volt Amperes Reactive (VARs) to the system to compensate for the lagging
power factor. Most electric utilities charge a penalty for poor system power factor, so
keeping the power factor above the required minimum value will result in a lower utility
bill and will also improve the voltage drop on the system.
Although both the current and the voltage oscillate sinusoidally in an AC circuit
they will not necessarily rise and fall simultaneously with each other in each circuit
element or the circuit as a whole. The current and voltage will oscillate with the same
frequency but they will (in general) be out of phase with each other,exception being
when the circuit is in resonance or if there is only resistor in the circuit.
The phase difference between two sinusoidal waveforms that have the same
frequency and are free of a dc component can be conveniently described as shown in
figure 1. It can be seen that the phase angle can be considered as a measure of the time
delay between two periodic signals expressed as a fraction of the wave period. This
fraction is normally expressed in units of angle, with a full cycle corresponding to 360°.
For example, in figure 1, where the voltage v1 passes through zero one-eighth cycles
before a second voltage v2, it leads v2 by (360°/8) or 45°.
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